watershedrestoration

watershedrestoration - NRM 4314 Watershed Planning V I I....

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Unformatted text preview: NRM 4314 Watershed Planning V I I. Watershed Restoration A. Introduction The whole area of watershed restoration is rather complex and success ratios are in general rather low. This is not to say we should not t ry restorative measures where needed but rather to emphasize that it is much easier and certainly less expensive to properly manage watersheds to prevent their deterioration in the first place. An example of the concepts involved in watershed restoration include the following: 1933: Fire denuded 7 square miles chaparral in Southern California-- 1 Jan 1934 Thunderstorm 500-1000 sec ft/sq mile $5,000,000 property damage 483 homes destroyed Adjacent similar- unburned areas 1 Prepared Dec 2008 NRM 4314 Watershed Planning Arroyo Seco 58 sec ft San Dimas Canyon 51 sec ft 1. Analyze the problems: a. causative agents. b. causative practices. c. damages. 2. Recognize the risks and limitations involved. 3. Design Solutions: a. storm return periods. 4. Plan for necessary structural maintenance. 5. Maximize recovery of natural or stable vegetation. B. General Considerations: One of the major items to consider in the planning of structural control measures is the runoff design. This is the peak runoff 2 Prepared Dec 2008 NRM 4314 Watershed Planning which the structure is constructed to handle. Calculation of peak runoff is determined by maximum storm intensities; return period probabilities, watershed areas, surface infiltration conditions and concentration times on the watershed. In general, structures designed for 25 year storms will have a 65% chance of survival during their first 10 years of use. This relationship, while not the best, is a reasonable compromise of cost and efforts compared to effectiveness. A. Structural Measures Definitions- The following definitions are based on material from Forest Service handbooks and current field terminology. Contour furrows- small level ditches made with plows or other light soil- moving equipment. They may be either continuous or discontinuous. 3 Prepared Dec 2008 NRM 4314 Watershed Planning Contour trenches- level ditches with cross-dikes or dividers at 50 to 65 foot intervals. Cross-dikes should extend about 2/3 the height of the trench. Trenches are larger than furrows and are normally constructed with heavier equipment. Although temporary in nature, they are longer-lasting than furrows. Diversion ditches- v-shaped or parabolic- shaped ditches laid out on a grade sufficient to transport water at a non- erosive rate away from critical areas and deliver it to a safe outlet. Drop-inlet spillway- an L-shaped pipe running under an earth dam and up into the storage pool behind the dam by means of a riser. Normally a drop-inlet is designed to accommodate a 25 year flood....
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watershedrestoration - NRM 4314 Watershed Planning V I I....

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